1. Filed of the Invention
The invention relates to a projection-microlithographic device having: a projection-microlithographic apparatus having: an illuminating device which itself comprises: a light source; a device which produces an image field configuration in a reticle plane; being designed such that an image field configuration is produced by a transformation of an initial rectangle having a long side and a short side with an aspect ratio greater than 1:5, said transformation being such as to keep the area and, in the direction of the long side of the rectangle, the total dimension constant; said generated image field configuration having at least two closed curves as periphery; a reticle holder which is designed to accommodate a reticle in the reticle plane and is movable in a scanning direction; and a projection objective imaging a pattern of the reticle located in the reticle plane onto a wafer plane.
In projection-microlithographic devices of this kind, the individual chips on the semiconductor wafer arranged beneath the projection objective are not exposed all over, but in a scanning movement in which both the reticle holder and the wafer holder are moved linearly at a generally constant speed and in synchronism with one another in a direction referred to as the scanning direction or y-direction. To achieve the largest possible dimension of the projection image in the direction at right angles to the scanning direction (x-direction) with the smallest possible objective diameters, image field configurations in the form of rectangular slots are normally used. The narrow side of these rectangles extends in the scanning direction.
2. Description of the Technical Field, Including Information Disclosed Under 37 CFR 1.97 and 1.98
It is known that the rectangular slot formation, with which the lenses of the projection objective are illuminated non-rotationally symmetrically, leads to non-rotationally symmetric imaging errors of the projection objective caused by lens heating and/or compaction. To eliminate and compensate for these non-rotationally symmetric imaging errors, a large number of approaches have been adopted, the principle of which is always the same: The imaging properties of the projection objective are to be symmetrized by means of a likewise non-rotationally symmetric measure complementary to the non-rotationally symmetric imaging errors of the projection objective. Such measures include non-rotationally symmetric heating or cooling, or mechanical deformation, of lenses. Apart from the fact that the subsequent compensation of non-rotationally symmetric imaging errors cannot always be performed optimally and often is accompanied by time characteristics that are difficult to manage, subsequent compensation normally involves a considerable additional outlay in terms of apparatus and corresponding costs.
An example of a projection-microlithograhic device of the type defined which attempts to compensate for non-rotationally symmetric imaging errors by means of additional measures is given in EP 0 823 662 A. Here, in addition to the light serving for illuminating purposes, additional light from other light sources is also sent through the projection objective; this additional light results in an overall symmetrization of the irradiation of the lenses of the objective, but is unsuitable for, or does not participate in, the exposure of the wafer. Obviously the additional light sources and optical components required for the input into the projection objective constitute a considerable cost outlay; moreover, the projection objective is subjected to additional, unnecessary thermal stress.
U.S. Pat. No. 5,473,410 also describes a projection-microlithographic device of the type referred to in the introduction. Here FIGS. 2A and 2B illustrate image field configurations in the form of a regular hexagon. This image field configuration has the following purpose: To minimize the projection objective diameter, the exposure of the chips on the wafer in the y-direction takes place not in one single scanning process but in a plurality of scanning processes performed in parallel to one another (xe2x80x9cstitchingxe2x80x9d process). To avoid exposure inhomogeneities at the edges, extending in parallel to the scanning direction, of the thus produced xe2x80x9cexposure stripsxe2x80x9d, the boundary lines of the image field configuration extend not in parallel to the scanning direction but obliquely thereto. A junction zone between adjacent xe2x80x9cexposure stripsxe2x80x9d therefore is overlappingly exposed in two consecutive scanning processes. In concrete terms, the aforementioned form of a regular hexagon is selected for the image field configuration for this purpose. However, a side effect of this image field configuration, which is not acknowledged in U.S. Pat. No. 5,473,410, is that the illumination of the projection objective takes place approximately rotationally symmetrically and therefore non-rotationally symmetric imaging errors are prevented from the start or occur only to a small extent. The described image field configuration is only suitable however for xe2x80x9cstitchingxe2x80x9d with overlapping exposure.
Also known are scanners with catadioptric or catoptric objectives which, due to beam shading of the mirrors and in accordance with the best image correction zone, image an extra axial ring sector. In such scanners, as in the case of so-called steppers operating with square image field configurations, the problem of asymmetric lens heating discussed here does not arise.
The aim of the present invention is to develop a projection-microlithographic device of the type referred to in the introduction, such that radiation induced, non-rotationally symmetric imaging errors of the projection objective are substantially avoided from the start, and that each individual chip on the wafer can be exposed in one single scanning process of the established type.
This aim is achieved, in accordance with the invention by a projection-microlithographic apparatus having: an illuminating device which itself comprises: a light source; a device which produces an image field configuration in a reticle plane; being designed such that an image field configuration is produced by a transformation of an initial rectangle having a long side and a short side with an aspect ratio greater than 1:5, said transformation being such as to keep the area and, in the direction of the long side of the rectangle, the total dimension constant; said generated image field configuration having at least two closed curves as periphery; a reticle holder which is designed to accommodate a reticle in the reticle plane and is movable in a scanning direction; a projection objective imaging a pattern of the reticle located in the reticle plane onto a wafer plane.
The device that produces the image field configuration is designed such that both the corners of the initial rectangle from which the image field configuration is generated and at least the outer corners of the image field configuration generated from this initial rectangle are located on a circular boundary line. This boundary line may coincide with a boundary line of a circular corrected image field of the projection objective or may define the outer boundary of an image field configuration lying within the circular corrected image filed of the projection objective. Therefore, the circular boundary line defined by the outer corners of the image filed configuration is not necessarily coincident with the boundary line of the corrected image field. Always, the image field configuration produced is located within the circular corrected image field.
The invention is based on the recognition that, whilst retaining an aspect ratio which applies to a classic rectangular slot formation, it is possible to achieve an image field configuration that is located more exactly in the peripheral region of the circular, corrected image field, so that the heat discharge is simplified and in this way heat induced imaging errors in the projection objective can be reduced.
In an embodiment of the invention the device that produces the image field configuration is designed such that the image field configuration fulfils the following conditions:
the device comprises fields which in the y-direction are separated from one another at least in parts by a free field and, in a manner at least roughly approximating rotation symmetry, are located in the peripheral region of the circular image field of the projection objective;
the integral of the quantity of light passing through the fields in the y-direction is constant over the entire extent of the image field configuration in the direction (x-direction) at right angles to the y-direction.
This embodiment of the invention departs, for the first time, from the hitherto widely applied principle that the image field configuration must consist of a cohesive surface. It is recognized that the exposure effect of a narrow scanner slot can also be equivalently replaced by a plurality of fields which are no longer directly joined up but are distributed in the peripheral region of the circular, corrected or correctable image field of the projection objective, such that they approximate rotation symmetry more closely than a narrow rectangular slot. The correction of the objective then need not include the unused regions. This at least roughly approximated rotation symmetry prevents radiation induced, non-rotationally symmetric imaging errors of the projection objective from the start, thus without compensatory measures. However, the exposure effect of the image field configuration according to the invention corresponds to that of the narrow, known scanner slot since, in the fields which constitute the image field configuration, the integral of the light quantity measured in the y-direction is constant over the entire extent of the image field configuration in the x-direction, i.e. corresponds to the width of the conventional slot. A disadvantage of the image field configuration according to the invention is that a slightly greater scanning movement of the reticle holder and of the wafer holder (overscan) is required, as the image field configurations according to the invention have a larger overall dimension in the y-direction than a conventional, narrow scanner slot.
If the illumination is homogeneous, a xe2x80x9cconstant integral of the light quantityxe2x80x9d in the y-direction can be achieved very easily by means of a constant integral of the dimension of the fields in the y-direction.
The detailed description sets forth image field configurations which fulfil the above mentioned purpose and can generally be achieved using relatively simple illuminating devices.
The detailed description discloses an illuminating device that corresponds closely to already existing illuminating devices, and thus entails a relatively minor modification of these already existing devices. It utilizes one or more glass rods as homogenizer in a manner similar to that described in U.S. Pat. No. 5,473,408. This is relatively cost effective and also opens up the possibility of retrofitting already existing devices.
The same applies to the means described in the specification, which permit a substantially loss-free, homogeneous illumination of the glass rods which constitute the device according to the invention that produces the image field configuration.
The detailed description sets forth a design of the device that produces the image field configuration comprising a prism honeycomb condenser, another low-cost option that can be retrofitted.